Not Applicable.
Not Applicable.
The present embodiments relate to radio frequency receivers, and are more particularly directed to a direct-conversion radio receiver (also known as a homodyne receiver).
Radio receivers have as their ultimate goal the function of receiving an incoming radio-frequency (xe2x80x9cRFxe2x80x9d) signal and outputting a corresponding baseband signal, typically so that the latter may be played through a speaker or the like. Two types of receivers have been developed which achieve these functions, one being a direct-conversion receiver and another being a heterodyne receiver. Generally speaking and as detailed below, each of these receivers serves the above-described goal using a different approach. By way of brief introduction to these approaches, the conversion of an incoming RF signal to a baseband signal is achieved by a direct-conversion receiver using a local oscillator which operates at the same frequency as the incoming RF signal, while a heterodyne receiver uses two or more local oscillators to progressively step down the incoming RF signal to intermediate frequency signals until the baseband result is reached.
While each of the two approaches described above has been used in various circuit implementations, each approach also involves different drawbacks, which also are introduced here. With respect to the direct-conversion receiver, it suffers from at least two drawbacks arising from its local oscillator signal. Specifically, in order to maintain good conversion gain, the local oscillator power is relatively large. As a result, some of this power leaks back toward the input of the system. This leakage has two negative effects. First, the leaking signal, which recall for a direct-conversion receiver is at the same frequency as the input signal, mixes with the input signal and continues through the receiver signal path leading to performance degradation in the receiver. Second, the leaking signal is connected back to the antenna which receives the system input and, as a result, the antenna in effect re-radiates this leaking signal into space which thereby may interfere with other receivers which are susceptible to the bandwidth of the local oscillator (and RF) signal. With respect to the heterodyne receiver, one of its drawbacks arises from its multiple bandpass filters. Specifically and as detailed below, these filters are generally included to remove so-called image signals corresponding to either the RF input signal or to any of the intermediate frequency signals. Using current methods for forming circuits, however, these bandpass filters cannot be formed in an integrated circuit; instead, they are required to be formed as discrete components. Thus, in forming a complete heterodyne receiver, each of these filters gives rise to a separate discrete component in the circuit chain, thereby increasing the number of integrated circuits involved. Consequently, cost and complexity are increased.
Two additional approaches have been implemented in the prior art to avoid certain of the problems mentioned above, but again each has its own corresponding drawbacks. In a first approach, the transmitted message signal is coded to ensure that no DC is present in the baseband message signal. On the receiving end, a high-pass filter is used in the signal path to reject any DC which does not meet the transmission restriction. However, this approach has at least three drawbacks. First, a constraint is imposed on the transmitted signal. Second, the baseband time constants are large and, thus, the capacitors in the receiver""s high-pass filter are correspondingly large. Third, this approach does not address the same problem of local oscillator re-radiation into space. In a second approach, the receiver finds the average of the demodulated signal constellation, predicts the DC leakage, and then removes the predicted value from the signal. This approach also has at least three drawbacks. First, this technique requires a large number of bits in the analog-to-digital converters at the baseband because the DC leakage determines the dynamic range of the receiver. Second, this approach requires high power dissipation in the analog-to-digital converters. Third, this approach also does not address the problem of local oscillator re-radiation into space.
In view of the above, there arises a need to address the limitation of prior art receivers by providing an improved receiver which reduces or eliminates the drawbacks described above.
In the preferred embodiment, there is a receiver comprising an input for receiving an RF signal centered at a first frequency and having a bandwidth. The receiver also comprises a first mixer for producing a first output signal. The first output signal results from mixing the RF signal with a signal having an energy spreading portion and a down-converting portion. Moreover, this first output signal comprises a self-mixing DC signal and a down-converted and energy spread RF signal. The receiver further comprises a second mixer for producing a second output signal by mixing a signal responsive to the first output signal with the energy spreading portion of the signal. The second output signal comprises two signals, namely: (1) a baseband signal responsive to the down-converted and energy spread RF signal; and (2) a portion of the spread DC signal. Other circuits, systems, and methods are also disclosed and claimed.